mehltretter



April 15, 1958 c. L. MEHLTRETTER' 2,330,941

ELECTROLYTIC PROCESS FOR MAKING PERIODIC ACID SOLUTIONS Filed Aug. 27, 1957 SODIUM SODIUM SULFURIC SULFATE HYDROXIDE WATER ACID INVENTOR CHARLES L. MEHLTRETTER ATTORNEYS ELECTROLYTIC PROCESS FOR MAKING PERIODIC Ailll) SOLUTIONS Charles L. Mehltretter, Peoria, Ill., assignor to the United States of America as represented by the Secretary of Agriculture Application August 27, 1957, Serial No. 680,610

4 Claims. (Cl. 204-82) (Granted under Title 35, U. S. Code (1952), sec. 266) A non-exclusive, irrevocable, royality-free license in the invention herein described, for all governmental purposes, throughout the world, with the power to grant sublicenses for such purposes, is hereby granted to the Government of the United States of America.

This invention relates to a process for the manufacture of acid solutions of sodium periodate by electrolysis. Such acid or neutralized solutions may be used directly as an oxidant in organic reactions such as the oxidation of polysaccharides to periodate oxypolysaccharides as described in U. S. Patent No. 2,713,55 3. The neutralized electrolysis solutions may also be used for the preparation of crystalline sodium metaperiodate as described in Inorganic Syntheses, vol. 1, pages 169-171 (first edition, McGraW-Hill Book Company,lnc., 1939).

In U. S. Patent 2,770,589 it has been disclosed that iodine in alkaline solution is completely converted by an electrolytic procedure to alkali salts of iodic acid. In this process the catholyte and anolyte solutions are alkaline and a graphite or carbon anode is used. When the electrolysis is extended under these conditions, periodate is not formed to any appreciable extent.- In this connection Hickling and Richards (J. Chem. Soc., page 260 (1940)) have shown that carbon anodes are not elfective for the conversion of iodate to periodate. However, electrolysis with lead dioxide anodes produces periodate from icdate in acid, neutral or alkaline solutions as Hickling and Richards found (J. Chem. Soc., pp. 257, 260 (1940) To achieve the electrolytic oxidation of iodine to peric-date by this procedure, obviously requires first the conversion of iodine to iodate using a carbon anode followed by oxidation of iodate to periodate in the presence of a lead dioxide anode.

The prior art also shows that iodine in hydrochloric acid solution may be electrolytically converted to iodic acid in a diaphragm cell by means of a platinum anode and a gold plated cathode. In a subsequent procedure the iodic acid is oxidized to periodic acid using a lead dioxide anode. This work is described by Willard and Ralston in Transactions of the Electrochemical Society, vol. 62, ages 239-254 (1932).

Willard and Ralston stated (page 251 of the above reference) that the two-stage procedure was employed because the oxidation of iodine directly to periodic acid using a lead dioxide anode was not practical due to corrosion of the electrode during the electrolysis.

It would be of economic advantage to oxidize iodine to periodate or periodic acid in a single-stage electrolytic process using the same anode and the same cathode throughout the electrolysis. it has now been found, according to the invention, that relatively low concentrations of iodine and the sodium hydroxide in aqueous solution can be electrolyzed Without difficulty in a diaphragm cell to periodic acid by continuing the electrolysis under acid conditions in the same .cell. In this novel process there is no need to change the electrodes before proceeding with the conversion of iodate to periodate as has been ice done in the prior art. However, when electrolytically oxidizing higher concentrations of iodine in sodium hydroxide solutions to periodate by means of a lead dioxide anode in a diaphragm cell containing an alkaline catholyte an insoluble coating of sodium paraperiodate forms on the anode during iodate production which seriously impairs the efficiency of the cell. Acidification to dissolve the paraperiodate before all of the iodine is oxidized to the intermediate sodium iodate, causes undesired precipitation of iodine in the anolyte. Continuation of the electrolysis in the alkaline cell to convert iodate to periodate causes the ceramic diaphragms to become coated with sodium paraperiodate. Precipitation of sodium paraperiodate also occurs in the body of the diaphragrns because of the alkalinity of both the anolyte and catholyte and results in-the blocking up of the pores of the diaphragm with a consequent increase in the resistance of the diaphragm. This increased electrical resistance combined with that of the coated lead dioxide anode causes an increase in potential difference across the cell which leads to a greater consumption of electrical energy per unit of periodate produced as well as to an increase in the amount of cooling required to maintain the cell at its efiicient working temperature. A further disadvantage is that on completion of the electrolysis, the sodium paraperiodate requires scraping from the diaphragms or solution in acid for its recovery.

It has now also been found, according to this invention, that the above disadvantage of using a lead dioxide anode'for the conversion of alkaline solutions containing higher concentrations of iodine to periodate can be obviated if an inorganic salt such as sodium sulfate or borax is dissolved in the anolyte prior to electrolysis to avoid producing a coating of insoluble sodium paraperiodate on the lead dioxide anode during iodate formation. Occasional addition of alkali to the anolyte may also be required during the electrolysis to keep the iodine in solution, and to maintain high conductivity of the cell. Electrolysisof iodine in alkaline solution with sodium sulfate or borax in the anolyte produces a continuous acid reaction at the surface of the anode which prevents the formation or appreciable formation of insoluble sodium paraperiodate at this surface, as shown in Table I.

TABLE I Efiecl of dissolved salts on coating of the lead dioxide anode during electrolytic oxidation of iodine t0 iodate at 40 C. and current density of 0.11 amperes per square centimeter. Four hundred milliliters of anolyre solution was used in each run.

Initial Iodine, sodium Dissolved salts, Anode grams hydroxgrams coating ide, grams 25 2O 0 None 25 30 0 50 30 0 50 20 0 50 20 20 (borax) None 50 20 2 (borax) None 50 2O 20 (Nazso None 50 20 2 Nelson Slight 60 20 10 (NazsO Slight 60 20 (NaiSOi) Slight 150 50 0 When necessary, alkali solution was added during the electrolysis to keep the iodine in solution.

Other alkalies such as potassium hydroxide, potassium carbonate, and sodium carbonate may be used in the anolyte, but it is preferred to use sodium hydroxide.

The quantity of sodium sulfate or borax used to prevent the formation of the coating of insoluble alkali paraperiodate on the lead: dioxide anode during the electrolysis is not critical.

It has further been found according to thisinvention that the disadvanta es of using ceramic diaphragms in combination with a lead dioxide anode for a one-stage conversion of iodine to periodic acid, can be obviated by the use of. an alkaline catholyte for the oxidation to iodate followed by an acid catholyte for the conversion of iodic to periodic acid. As soon as essentially all of the iodine has been converted to sodium iodate the akaline catholyte is flushed from the'diaphragms with water followed by 10 percent sulfuric acid. The sulfuric acid is allowed to flow continuously or intermittently through the diaphragms tokeep the catholyte acid for the remainder of the electrolysis. The anolyte soon becomes acid automatically through electrical transfer of sulfate ions from a the catholyte to the anode and by osmosis of acid from the catholyte. To hasten acidification of the anolyte a small amount of sulfuric acid may be introduced. The complete electrolytic oxidation is thus essentially that of iodine to iodate and of iodic acid to periodic acid which is extremely soluble in water and avoids coating of the diaphragms with insoluble alkaline sodium paraperiodate.

Accordingly, the present invention provides an improved process for the manufacture of acid solutions of sodium periodate by the electrolysis of alkaline solutions of iodine to iodate with continuation of the electrolysis under acidic conditions in the same cell using the same anode and the same cathodes to produce acid solutions of sodium periodate. The anolyte also contains sodium sulfate or borax in solution to prevent coating of the anode during the electrolysis to iodate as described above.

In general, the invention is carried out by passing an electric current through a cell containing an anolyte and a catholyte separated by a porous diaphragm, the catholyte comprising an aqueous solution of alkali, preferably sodium hydroxide, in which is immersed a cathode capable of resistance to corrosion by alkali and by acid in a reducing environment and the anolyte comprising an aqueous solution of elemental iodine and alkali, preferably sodium hydroxide, in which is immersed a lead dioxide anode, the anolyte containing, in addition, a suflicient amount of sodium sulfate or borax to prevent formation of a coating of insoluble alkali paraperiodat-e on the lead dioxide anode during the electrolysis. The electric current is passed through the cell at a temperature of about from 5 C. to 60 C., utilizing a current density of about from 0.05 to 0.3 ampere per square centimeter of the lead dioxide anode surface, until essentially all of the iodine has been converted to alkali iodate. Thereafter, the catholyte and anolyte are acidified and the electrolysis -continued under such acidic conditions to convert the alkali iodate to alkali periodate.

Preferably lead dioxide anodes and graphite cathodes are employed. A cell suitable for carrying out the process of the present invention is shown in the accompanying drawing. It shows the flow of materials in a complete process wherein iodine is oxidized electrochemically to periodic acid. Resublimed iodine or commercial crude iodine is fed into mixer 1 where it is reacted with the appropriate amount of sodium hydroxide in an aqueous solution supplied from tank 2. Aqueous sodium sulfate may he added to the mixer from tank 3. From mixer 1, the solution or mixture is fed to the electrolytic cell and is the anolyte 4. The cell is provided with a lead dioxide anode 5 immersed in the anolyte and graphite cathodes 6 and7 situated inside the porous diaphragms 8 and 9 containing catholytc solutions 32 and 13. The alkaline anolyte solution or mixture is oxidized to sodium iodate at the lead dioxide anode using an alkaline catholyte solution in the cliaphragms 8 and 9 supplied from electrolysis alkali may be added to the anolyte from tank If necessary during the 2. When iodine is essentially completely converted to iodate the diaphragms are flushed with water from tank 19 to remove most of the alkaline solution which operation is followed by flushing with dilute sulfuric acid from tank 11. The electrolysis is continued during these operations. The acidity of the catholyte is maintained during the remainder of the electrolysis to periodic acid by slow passage of sulfuric acid through the diaphragms from tank 11. Catholyte effluents 15 and 16 may be saved for recovery of alkali for recycling or run to the sewer. On completion of the oxidation to periodic acid the anolyte solution is passed through a filter 14 to remove particles of lead dioxide that usually shed from the anode. To control its temperature the cell is cooled by means of coils inside the anolyte chamber through which cold water is passed. The anolyte is well stirred to prevent polarization at the anode. It may also be agitated by a circulating pump system in place of stirring. A rotating anode may also, be used for this purpose.

The process may be carried out either batchwise or by continuous or semi-continuous operation. The filtered anolyte containing periodic acid may be used directly for the oxidation of organic compounds, and, in particular, starch to periodate oxystarch. It may also be neutralized with sodium hydroxide and subjected to evaporative concentration to cause crystallization of sodium periodate.

The conditions of electrolysis used in this invention are constant stirring of the anolyte to maintain homogeneity and a temperature maintained within the range 5 C. to C. The amount of current required may vary over a wide range. It has been found that a current density of from 0.05 to 0.30 ampere per square centimeter of anode surface gives good results. Low current densities favor higher current efficiencies. However, to shorten the time of electrolysis it is desirable to use a large anode surface and a high current density The concentration of alkali and of sulfuric acid in the catholyte is not critical and may vary between wide limits. Graphite is used as the cathode material because of its resistance to corrosion by alkali or acid in a reducing environment, availability and low cost. However, the cathode material is not limited to carbon or graphite.

The following examples illustrate the invention. In each example an electrolytic cell was employed consisting of two diaphragms of Norton Company alundum No. 19318 porosity RA84 in an 800-1111. beaker. Other acid and alkali resistant materials, such as asbestos may also be used to separate the anode and cathode compartments. The cathodes were graphite rods of about 24 square centi' meter surface area immersed in the catholyte throughout the electrolysis. The concentration of alkali in the catholyte was from 2 to 30 percent sodium hydroxide. The Catholyte surface was maintained higher than the anolyte surface to prevent loss of anions by osmosis through the diaphragms into the catholyte. The anode was a lead dioxide coated lead sheet, 66 square centimeters of which were immersed in the anolyte.

EXAMPLE 1 The two diaphragxns or cathode compartments of a three-compartment cell were each charged with 30 ml. of 5 percent sodium hydroxide solution. T o the anode compartment was introduced the anolyte prepared by adding 50 grams of iodine to a solution of 20 grams of sodium hydroxide and 10 grams of borax in 400 ml. of distilled water. The anolyte mixture was vigorously stirred during the electrolysis at a temperature of 40 C. maintained by outside cooling of the cell while passing a current of 7.5 amperes through the cell. At various times during the electrolysis, several milliliters of 40 percent sodium hydroxide were added to the anolyte to decrease the iodine color. After 6% hours essentially all of the iodine was oxidized to sodium iodate. The diaphragms were then fiushed'with distilled water to remove most of the alkali solution which was replaced with 10 percent sulfuric acid. The catholyte was then maintained acid by fiow of sulfuric acid. The anolyte was also acidified by addition of 1 ml. of concentrated sulfuric acid. Electrolysis was continued for 6 hours (total 12% hours) to convert essentially all of the iodate in acid solution to periodic acid. Analysis showed that 72.8 grams of periodic acid was present which is a yield of 96 percent of theory.

EXAMPLE 2 Using the same set-up as in Example 1, and an anolyte prepared by adding 150 grams of commercial crude iodine to a solution of 50 grams of sodium hydroxide and grams of sodium sulfate in 400 ml. of distilled water, electrolysis at 7.5 amperes was carried out for 22% hours at 50 C. to convert all of the iodine to iodate. The electrolysis was continued for 17 /2 hours under acidic conditions as described in Example 1 to oxidize all of the acid solution of sodium iodate to periodic acid which was formed in solution in a yield of 95 percent of theory.

The anolyte solution was filtered from lead dioxide particles and could be used directly for the oxidation of starch to periodate oxystarch or neutralized and -concentrated to yield crystalline sodium metaperiodate.

EXAMPLE 3 Using the same set-up as in Example 1 and an anolyte prepared by adding 50 grams of commercial crude iodine (99.7 percent pure) to a solution of 20 grams of sodium hydroxide and 2 grams of sodium sulfate in 400 ml. of distilled water, electrolysis was carried out at 7.5 amperes and 40 C. for a total of 13 /2 hours. After 7 hours of electrolysis essentially all of the iodine was oxidized to iodate. Continuation of the electrolysis under acidic conditions as in Example 1 for 6 /2 hours produced 73.1 grams of periodic acid in solution, which is a yield of 97 percent of theory.

EXAMPLE 4 Using the same set-up as in Example 1 and an anolyte prepared by introducing 25 grams of commercial crude iodine to a solution of 20 grams of sodium hydroxide in 400 ml. of distilled water, electrolysis at 7.5 amperes was carried out at 30 C. for 5 hours for conversion of essentially all of the iodine to iodate. The electrolysis was continued under acidic conditions as described in Example 1 for 2 /2 hours for essentially complete oxidation of the acid solution of sodium iodate to yield a solution of periodic acid. The production of periodic acid was 94 percent of theory.

Having thus disclosed my invention, I claim:

1. A process for preparing an alkali periodate comprising passing an electric current through a cell containing an anolyte and a catholyte separated by a porous diaphragm, said catholyte comprising an aqueous solution of alkali in which is immersed a cathode capable of resistance to corrosion by alkali and by acid in a reducing environment, said anolyte comprising an aqueous solution of elemental iodine and alkali in which is immersed a lead dioxide anode, said anolyte containing, in addition, a sufiicient amount of an inorganic salt selected from the group consisting of sodium sulfate and borax to prevent the formation of a coating of insoluble alkali paraperiodate on said lead dioxide anode during the electrolysis, said electric current being passed through said cell at a temperature of about from 5 C. to C. and utilizing a current density of about from 0.05 to 0.3 ampere per square centimeter of said lead dioxide anode surface until essentially all of the iodine has been converted to alkali iodate, thereafter acidifying the catholyte and anolyte and continuing the electrolysis under such acidic conditions to convert the alkali iodate to alkali periodate.

2. The process of claim 1 wherein the alkali is sodium hydroxide.

3. The process of claim 1 wherein the inorganic salt is sodium sulfate.

4. The process of claim 1 wherein the inorganic salt is borax.

References Cited in the file of this patent Trans. of the Electrochem. Soc., vol. 62 (1932), by Willard et al., pp. 239 to 254. 

1. A PROCESS FOR PREPARING AN ALKALI PERIODATE COMPRISING PASSING AN ELECTRIC CURRENT THROUGH A CELL CONTAINING AN ANNOLYTE AND A CATHOLYTE SEPARATE BY A POROUS DIAPHRAGM, SAID CATHOLYTE COMPRISING AN AQUEOUS SOLUTION OF ALKALI IN WHICH IS IMMERSED A CATHODE CAPABLE OF RESISTANCE TO CORROSION BY ALKALI AND BY ACID IN A REDCING ENVIRONMENT, SAID ANOLYTE COMPRISING AN AQUEOUS SOLUTION OF ELEMENTAL IODINE AND ALKALI IN WHICH IS IMMERSED A LEAD DIOXIDE ANODE, SAID ANOLYTE CONTAINING, IN ADDITION, A SUFFICIENT AMOUNT OF AN INORGANIC SALT SELECTED FROM THE GROUP CONSISTING OF SODIUM SULFATE AND BORAX TO PREVENT THE FORMATION OF A COATING OF INSOLUBLE ALKALI PARAPERIODATE ON SAID LEAD DIOXIDE ANODE DURINGG THE ELECTROLYSIS, SAID ELECTRIC CURRENT BEING PASSED THROUGH SAID CELL AT A TEMPERATURE OF ABOUT FROM 5*C. TO 60*C. AND UTILIZING A CURRENT DENSITY OF ABOUT FROM 0.05 TO 0.3 AMPERE PER SQUARE CENTIMETER OF SAID LEAD DIOXIDE ANODE SURFACE UNTIL ESSENTIALLY ALL OF THE IODINE HAS BEEN CONVERTED TO ALKALI IODATE, THEREAFTER ACIDIFYING THE CATHOLYTE AND ANOYLTE AND CONTAINING THE ELECTROLYSIS UNDER SUCH ACIDIC CONDITIONS TO CONVERT THE ALKALI IODATE TO ALKALI PERIODATE. 